Method and means for indicating changes in the composition of a gas



Jan. 15, 1952 T. D. MORGAN 2,582,647 METHOD AND MEANS FOR INDICATINGCHANGES IN THE COMPOSITION OF A GAS Filed Jan. 4, 1949 l I 1142l||||||||| 44 INVENTOR.

A 7' TORNEVS Patented Jan. 15, 1 952 METHOD AND MEANS FOR INDICATINCHANGES IN THE COMPOSITION OF AGAS Thomas D. Morgan, Bartlesville, kla.,assignor to l'hillips Petroleum Company, a corporation oi DelawareApplication January 4, 1949, Serial No. 69,226

13 Claims.

This invention relates to a method of determining changes in thecomposition ot a gas and apparatus for carrying out the method. In onespecific aspect it relates to determining diflerences in composition ofa gas by changes in the ionization of the gas as determined by theamount of current carried by positive ions in said 88. In anotherspecific aspect it relates $0 bombarding molecules of gas by low speedelectrons to iorm positive ions. conducting a current through saidpositive ions and measuring the amount of said current.

Claims relating to a specific means for regulating the gas pressure in achamber comprising evacuating means, gas supply means, conduit means,pressure regulating valve means, and a restricted orifice forcontrolling gas flow in the conduit have been made the sub ect matter ofmy copending divisional application, Serial No. 240,244, filed August 3,1951.

It has been observed that the sum of the ions in a mass spectrum variesfrom one gas to another. -This sum is proportional to the total numberof positive ions formed when a gas is bombarded by low speed electrons.

Many complicated means have been proposed in prior art for determiningwhen the composition of a gas, such as a stream of gaseous productcoming from a process in a plant. changes in composition, in order tomonitor the process by indicating that the process is continuing asplanned. When the composition of the gas varies, the process involved isnot operating properly, and changes are made to restore the operation tonormal. Prior art devices testing the stream of gas generally haverelied on analysis of the gas by means of spectro-graphic analysis,chemical analysis, analysis by mass spectrometer, or some otherrelatively complicated and slow form of analysis.

The present invention gets away from such complicated and expensivemeans of analysis by relying on the simple fact that each gas forms adiiferent number of positive ions when bombarded under the sameconditions by low speed electrons. The present invention also avoidsindicating false changes in the composition of the gas caused byirregular thermal cracking of the original gas into other gases bycontact with the filament from which the low speed electrons are beingsupplied. This is avoided by means of a novel arrangement of thefilament, grids and plate so that the gas entering the test apparatus issubjected to low speed electron bombardment and the current is set up inthe positive ions 01 the gas between a first grid and the plate beforethe gas passes through said first grid into contact with the heatedfilament. The gas which has contacted the heated filament and becomethermally cracked is removed from the system from 2 the other side ofthe grid from the gas which is conducting the indicating current.

One ob ect of my invention is to provide an improved method of detectingand indicating dinerences in the composition of a gas.

Another object is to provide improved apparatus for carrying out such amethod.

Another object is to provide such a method and apparatus in which thepositive ions conducting the current are bombarded by low speedelectrons without coming in contact with any heated element which wouldchange their composition by thermal cracking or the like.

Another object is to provide an improved method and means of detecting adifierence in the composition of the gas by measuring an ionizationcurrent in said gas in an improved ionization tube.

Another object of the invention is to detect and indicate changes in thetube of a, gas stream and thereby to qualitatively determine the gas ,3causing the deviation from the indication given by a gas or standardcomposition.

Another object is to indicate changes in the composition of a gas toprovide a monitor which indicates when changes in a process of producinga gas should be made.

With a pure gas an object of the invention is to give an indication ofthe presence of impurities in said gas.

With binary mixtures of gases an object of the invention is to analyzethe mixture to determine the exact percentage of each gas present.

With mixtures 01 known gases an ob ect of the inventionis to analyze themixture to determine the exact percentage of each gas present.

Another object is to provide an improved gas ionization tube which isrugged, simple in composition, is not efiected by products of thermalcracking, and which is relatively cheap and easy to operate.

Another object is to provide a suitable means of supplying gas to such atube at a regulated pressure. I

Numerous other objects and advantages will be apparent upon reading theaccompanying specification claims and drawings.

Figure 1 is an elevational view with parts in cross section of anionization tube and means to supply the same with a stream of gas to betested embodying the present invention, along with a wiring diagram forthe same.

Figure 2 is an elevational view of a similar system employing the sametype of ionization tube but having amodified system for supplying gas.

In Figure 1 a device tor indicating changes in the composition of a gasby ionization currents generally designated as 3 is attached to a pipe 4containing a flowing stream of gas which passes out through pipe 6. Asmall amount of this gas is constantly being drawn into instrument 3through pipe 1 under the control of a constant pressure regulating valve8 which is designed to maintain the pressure in space 9 at asubstantially constant pressure.

Also connected to apparatus 3 is an exhaust pipe II by which theinterior of 3 is exhausted to a substantially constant pressure ofapproximately 10- mm. of mercury by means of any suitable vacuum pumpl2. Vacuum pump I2 is regulated by constant pressure regulator l3 whichmay be of any usual type, regulator l3 being controlled as shown by thedotted lines between the pressure regulator l3 and tube Vacuum pump |2discharges through line l4 to the atmosphere. In many installations withvarious types of vacuum pumps l2, it has been found that one pump alonedoes not provide a constant enough vacuum and it is customary to useseparate pumps (not shown) in series in pipe H with pump |2. Forexample, pump l2 could be a VMF-20 oil diffusion pump and downstream inline l4 (not shown) and connected in series therewith a VMF-5 oildiffusion pump and then a Welch duo seal force pump. Also in line Hbefore entering pump l2 an ice trap (not shown) may be provided.Chambers containing desiccant (not shown) could supplement the ice trapif the gas being tested contains water vapor. As such elaborations onvacuum pump |2 are well known in the art of evacuatin vessels it is notbelieved desirable to confuse the invention by elaborating on the typeof vacuum pump system used at l2, it being understood that any suitablesystem known to the prior art for producing a vacuum in the neighborhoodof mm. of mercury which can be regulated to give a relatively constantpressure in tube II is satisfactory in the practice of the presentinvention.

The pressure in the space 9 is of course considerably higher than 10-mm. of mercury because it is difilcult to find a regulating valve 8which is accurate at such a pressure, however any system known to theprior art for getting the pressure in line 4 down to the desiredpressure in space is may be employed. I prefer however to employ arestricted orifice H in a thickened portion l8 of tube 1 so that thepressure in space 9 may be in the neighborhood of 2 mm. of mercury whilethat in space l6 and space H is approximately 10- mm. of mercury.

While other shapes of apparatus may be employed provided the principleof operation is the same, it is preferred to use as the ionization tubeIQ of apparatus 3 a cylindrical tube preferably made of glass. A plate2| is provided inside the tube, which plate should have a considerableextent and preferably extends entirely around the tube to serve as theion collector plate. I find the best way of forming this plate is tovaporize and deposit platinum on the glass surface of tube l9. Metallicplate 2| is connected to wire 22. A source of negative electrons isprovided and is shown in the form of a naked tungsten metal filament 23;Other metal filaments may be employed provided they are not poisoned bygases. Obviously other types of heated cathodes may be employed, forexample the filament 23 may be sheathed in a porcelain tube (not shownwhich porcelain tube is heated to incandescence and this porcelain tubemay have metallic substances such as tungsten metal deposited thereon.However it is not believed necessary to go into detail on the electronemissive cathode 23 as such cathodes are well known in the electron artand any type producing slow electrons is satisfactory. By slow electronI mean an electron having a velocity within the ranges understood bythose skilled in the art which upon being accelerated by grid 24 wouldbe suitable for causing the formation of positive ions in the gas in thepressure of 10'- mm. of mercury. I have chosen a pressure of the manitude of about 10- mm. of mercury because at that pressure positiveions are more easily and more uniformly formed by electron bombardment,but those skilled in the art can easily predict the amount this pressurecan be changed without departing from the present invention, for as longas the same type of operation occurs the process is unchanged and thepresent invention is being employed.

Between the electron source 23 and the positive collection plate 2| Iprefer a positive charged grid generally designated as 24. Grid 24 ispreferably composed of two elements, the first bein a helical wire 26suitable for degassing the metallic parts of tube l9 as will beexplained later and a gauze wire cylinder 21 maintained at the samepotential by wire 28. Tube 21 extends entirely around filament 23 but toavoid confusion, has not been drawn except where it is in section, asdrawing all of the wires in cylinder 21 in back of filament 23 wouldmake it almost impossible to see filament 23.

While the particular mode of supporting grid 21 is immaterial I preferto imbed the lower end in an annular glass boss 29 formed in the wall oftube l9 and the upper end of 21 is similarly imbedded in the lower endof tube 3| which forms an extension of tube II. It will be obvious thatgas coming from l6 passes in space 32 between plate 2| and grid 21before passing through grid 21 and out tube 3| and tube II, and it willalso be obvious that any of the gas which contacts heated filament 23and is thermally cracked thereby will pass up tube 3| without reachingplate 2|.

The Wiring diagram will now be explained. In order to have a referencepoint let us say that wire 33 is at an intermediate potential. Filament23 is at the same potential at the top of the filament but there is adifference of about the magnitude of 5 volts provided by battery 34 andrheostat 36 between the ends of the filament which are connected towires 31 and 38. Most of the current from battery 34 is forced to gothrough wires 31 and 38 by relatively high resistances 39 and 4|, whichhowever allow wire 33 to be at the same average potential as filament23.

Plate 2| is preferably maintained at a lower potential than filament 23,but this is not essential as long as plate 2| is at a considerably lowerpotential than grid 21 in order that positive ions created in the spacebetween grid 21 and plate 2| will be urged toward plate 2| byelectrostatic forces and will conduct a current thereby to plate 2|. Asshown in the drawing it is preferred to provide a battery 42 which willkeep plate 2| about 12 volts or so below the potential oi filament 23and about or volts below the potential of grid 21.

In the process of maintaining plate 2| at this potential it is necessaryfor current to flow from battery 42 through a milliammeter 43 to wire 22because a current is being conducted by positive electrons between grid21 and plate 2 I. Milliamthis current.

especially filament 23. grid 26 and grid 21.

The main source of difference in potential between plate 21 and grids 26and 21 is battery 44, which is connected through instrument 46 with wire41 leading to grid 26 and through wire 28 to grid 21. Battery 44 ispreferably controlled by rheostat 45 to maintain a constant grid tofilament current. Battery 44 is preferably in the neighborhood of 75volts and in combination with battery 42 provides about 8'! voltspotential drop between grid 21 andplate 2|. During normal operationmilliammeter 46 indicates a filament to grid current of aboutmilliamperes.

After relatively long continued use, and after any time the vacuum intube l3 has been broken and air or other gas under substantiallyatmospheric pressure has been in space 32, I- have found that gasbecomes occluded in the metal part: thereupon becomes desirable to driveoil this occluded gas because such gas tends to reduce the accuracy ofthe readings. For this purpose a wire 46 is connected to helicalfilament 26 and through battery 46, switch Ii and wires 62 and 41 to theother end of grid 26. When switch 6| is closed, grid 26 is heated up bya suitable current from battery 43. I prefer battery 46 to be about 6volts and pass a current of 4 or 5 amperes through grid 26.

It should be realized that while for illustrative purposes specificvoltages, potentials and the like have been given that the invention isnot limited thereby or thereto as any electronic engineer can modifythese voltages and other electrical relationships within suitable rangesand still practice the present invention.

In Figure2, tube I3 is exactly the same as in Figure 1 and therefore isgiven the same reference numerals. The same is true of wires 22, 41. 46and milliammeters 43 and 46 and the electrical circuit between saidmilliammeters and said tube. Pump l2, regulator l3 and exhaust pipe l4are also the same and so are pipes 4. 6, 1 and regulator 8.

Between regulator 6 and restricted orifice l1 however a different formof apparatus has been inserted in order to regulate the pressure in tubel9 to a pressure in the neighborhood of 10- mm. of mercury. Regulator 6discharges into pipe 53 at a constant pressure. A clock 64 drives avalve operating mechanism 66. This valve operating mechanism 66 firstopens valve 61 by a means of solenoid valve operator 66. Valve 63 isclosed. Valve 51 is reclosed and then valve 63 is opened by solenoid 6|controlled by clock 64 whereupon the gas in tube 62 expands from itspressure of several mm. of mercury and passes through pipe 63 intostorage chamber to provide a source of gas. Valve 63 is then closed. Gasfrom 64 leaks uniformly through orifice l1 into tube I9 where theoperation becomes the same as in Figure 1.

When intermittent operation is desired valve 65A is actuated along withvalve 61 to open flow through pipe 65B by closing switch "D so thatsolenoids 65C and 66 are in parallel and operate in unison. While 62fills through valve 61, parts 63 and 64 are rapidly evacuated throughpipe 653 by pump l2.

While the size of orifice l1 may be varied considerably I have obtainedsatisfactory results by making this orifice by punching a hole about 20microns in diameter in a 0.0003" platinum plate with a sharpened needleand mounting the plate in the glass tube (not shown), or by forming afine hole in the glass as shown.

Operation The operation of Figure 1 will now be described. Vacuum pumpI2 is started up and the pressure in space 32 of tube It is lowered tosomewhere in the neighborhood of 10- mm. of mercury. Switch ii is thenclosed a suitable length of time, say about 15 minutes, during whichtime grid 26 becomes incandescent driving any occluded gases, out ofitself, out of filament 23, out of grid 21 and plate 2|. As fast asthese occluded gases are driven on they are removed through pipe byvacuum pump l2, switch 5| is then opened and pump 12 continues to pump.out tube l9, grid 26 cools down and the system comes into a relativelystable state in a few minutes.

In this relatively stable state a relatively uniform representativesample of gas from pipe 4 is passing through regulator 6, throughorifice l1, through space 32 between plate 2| and grid 21. through themeshes of grid 21 and on out through pipes 3| and II to vacuum pump l2.

Filament 23 is heated by battery 34 and is emitting slow electrons. Acertain constant proportion of these slow electrons are accelerated by,attracted to and collide with positively charged grids 26 and 21, but arelatively con- ,stant number of electrons pass through grid 21 out intospace 32 where they collide with gas molecules forming positive ions inthe process of such collisions. At the selected predetermined uniformpressure, such as 10- mm. of mercury, the number of positive ions formedvaries with the type of gas molecules struck by the electrons, while acertain constant proportion of these electrons are attracted by and fallback into grid 21. Therefore for any constant pressure in space 32 thereis a definite number of positive ions formed per second depending on thecomposition of the gas in the space.

These positive ions in space 32 are in an electrostatic field becausegrid 21 is charged at a higher potential (preferably about to voltshigher) than plate 2 I. The force of this electrostatic field moves thepositive ions steadily over to plate 2| and because of this ion currentthe battery 42 is caused to supply an equivalent current throughmilliammeter 43 where it is indicated. Milliammeter 43 can be arecording milliammeter if desired, and in fact can be used to controlmachinery (not shown) which exists in the prior art, which can be sooperated, by which the milliammeter 43 can control the process (notshown) producing the gas in pipe 4. However I prefer to merely indicatethe change in the composition of gas, and then have someone investigatewhy this change is taking place and make suitable adjustments to restorethe former composition of gas in pipe 4. Modern electronic controls arequite sensitive and can be operated by milliammeters, or by low levelcurrents.

While milliammeter 43 is indicating the rate of ionization of the gas,milliammeter 46 is merely indicating the grid to filament current intube I9 and should read a steady value in order to provide accurateoperation of the device. Any desired steady grid to filament current canbe maintained by adjusting rheostat 46 from time to time. While this canbe done manually, it obviously is preferable to substitute one of themany constant current controls now on the market, as such an automaticelectronic current control circuit (not shown) can do a better job ofregulating the current in wire 41 to about 5 7 milliamperes than manualoperation of rheostat I.

The operation of Figure 2 is very similar except for the means oflowering the pressure from line 4 down to that necessary in tube i8. Inmany plant operations the pressure in line 4 is too high to permit asingle stage reduction of pressure by regulator 8, so in Figure 2 thepressure having been made constant by regulator 8 in pipe 53, a definitevolume in pipe 82p: this gas under the pressure in pipe 63 is collectedin pipe 62 by opening valve 51 and closing the same and then this gas ispassed to storage vessel 64 at a low pressure by opening valve 59 andclosing the same. The gas in vessel 64 is at a substantially uniformpressure during the cycles employing valves 51 and 59 and by the timethis gas passes through orifice l1 into chamber 19 it is at asubstantially constant reduced pressure as desired. 1

It should be noted that after the gas passes through positively chargedgrid 21, contacts heated filament 23 and is thereby cracked to form adifierent gas, that any positive ions formed in this different gas byelectrons emitted from filament 23 cannot get through grid 21 becausethey are repelled by the positive charge on the grid, and therefore theymust pass up tube 3| and be exhausted by pump l2. Therefore it does notmatter that filament 23 changes the composition of the gas by crackingbecause grid 21 protects the instrument from any variation inmeasurements due to the same. Other irregularities in ionization of thegas, such as thermal cracking and formation of ions on the surface ofthe filament itself, make the complete removal of gas passing close tothe filament essential in order to achieve a high degree of accuracy.

EXAMPLE The readings of milliammeter 43 will vary from instrument toinstrument, the relative readings will be different for the difierentgases and about of the order set forth in the following table. Thistable is made at a constant pressure of 10- mm. of mercury. Obviouslyall comparative tests should be made at the same constant pressure, asvarying the pressure varies the amount of ionization. This selectedconstant pressure can be chosen from a quite wide range of pressureshowever, but preferably is chosen from the best range of operation,which runs from 10- mm. to 10- mm. of mercury, and I prefer theneighborhood of 10- mm. of mercury as the best range to operate it. Oncethe pressure is selected it is maintained constant as closely as it ispossible to do so.

Table It should be obvious that if only butene-l and butadiene arepresent in tube 4 that a reading 8 intermediate 2.60 and 1.26 will begiven at milliammeter 43. By tests of known amounts of these gases acalibration table can be prepared from which the reading can betranslated into .definite percentages of each component. Other similaruses are obvious with more than two gases where only one gas varies.Other uses are suggested to those skilled in the art by this example.

While I have shown and described a certain arrangement of parts forcarrying out the process of my invention and have described certainspecific processes, these have been for purpose of illustration andobviously my inventionis not limited thereto but instead is as definedin the following claims.

Having described my invention, I claim:

1. The process of indicating a change in the composition of a gas whichcomprises passing a sample of the gas at a constant low pressure througha zone where it is subjected to a bombardment of slow electrons to forma first group of positive ions at a rate dependent upon the compositionof the gas, collecting said first group of positive ions on a.negatively charged plate, measuring the electric current resultingtherefrom, and protecting said first group of positive ions fromcontamination with a second group of positive ions formed from suchportions of said gas as come in contact with the source of suchelectrons and crack thereon by repelling said second group of positiveions with a positively charged screen.

2. A gas analyzer comprising in combination a chamber, means for takin asample of a stream of gas, means for reducing and keepin the pressure ofsaidsample at a constant value and passing itinto said chamber, anexhaust conduit connected to said chamber, means to exhaust said exhaustconduit and thereby exhaust said chamber, a cathode in said chamber,means to heat said cathode so it will emit electrons, a screensurrounding said cathode .and connected to the walls of said exhaustconduit and said chamber so as to isolate said cathode and said exhaustconduit from the rest of said chamber. a positive ion collecting elementin said chamber on the other side of said screen from said cathode,means to heat said cathode, screen and element to drive out any gasesoccluded therein, means to maintain said screen positive relative tosaid cathode and means to maintain said element negative relative tosaid screen.

3. A gas analyzer comprising in combination a chamber, means for takinga sample of a stream of gas, means for reducing and keeping the pressureof said sample at a constant value and passing it into said chamber, anexhaust conduit connected to said chamber, means to exhaust said exhaustconduit and thereby exhaust said chamber, a cathode in said chamber,means to heat said cathode so it will emit electrons, a screensurrounding said cathode and connected to the walls of said exhaustconduit and said chamber so as to isolate said cathode and said exhaustconduit from the rest of said chamber, a positive ion collecting elementin said chamber on the other side of said screen from said cathode,means to maintain said screen positive relative to said cathode andmeans to maintain said element negative relativeto said screen.

4. In a gas analyzer an evacuated chamber, and means to regulate thepressure in said chamber comprisin vacuum pump means for evacuating saidchamber to a predetermined pressure, and means supplying gas to maintainsaid pressure comprising a source of said gas at greater thanatmospheric pressure, a conduit connecting said source and said chamber,a constant pressure regulating valve in said conduit receiving gas fromsaid source and passing it under reduced pressure through said conduit,and a restricted orifice controlling flow from said conduit into saidevacuated chamber, said chamber having a cen tral depending exhaust ductand an inlet duct disposed to discharge against said exhaust duct, ascreen sealed to and completely surrounding said exhaust duct andforming said chamber into two portions isolated by said screen, acathode on the exhaust duct side of said screen, and a film of depositedmetal on the walls of said chamber on the inlet side of said screen.

5. An ionization tube comprising in combination a glass chamber having acentral depending exhaust duct and a radial inlet duct disposed todischarge against the side of said exhaust duct,

-a gauze screen sealed to and completely surrounding said exhaust ductand forming said chamber into two portions isolated by said screen, acathode and screen heating means on the exhaust duct side of saidscreen, and a film of deposited platinum metal on the walls of saidchamber on the inlet side of said screen.

6. An ionization tube comprising in combination a glass chamber havingacentral depending exhaust duct and an inlet duct disposed to dischargeagainst the side of said exhaust duct, a screen sealed to and completelysurrounding said exhaust duct and forming said chamber into two portionsisolated by said screen, a cathode and screen heating means on theexhaust duct side of said screen, and a film of deposited platinum metalon the walls of said chamber on the inlet side of said screen.

7. 'An ionization tube comprising in combination a glass chamber havinga central depending exhaust duct and a radial inlet duct disposed todischarge against the side of said exhaust duct, a gauze screen sealedto and completely surroundingsaid exhaust duct and forming said chamberinto two portions isolated by said screen, a cathode on the exhaust ductside of said screen, and a film of deposited platinum metal on the wallsof said chamber on the inlet side of said screen.

8. An ionization tube comprising in combination a chamber having acentral depending exhaust duct and a radial inlet duct disposed todischarge against said exhaust duct, a gauze screen sealed to andcompletely surrounding said exhaust duct and forming said chamber intotwo portions isolated by said screen, a cathode and screen heating meanson the exhaust duct side of said screen, and a film of deposited metalon the walls of said chamber on the inlet side of said screen.

9. An ionization tube comprising in combination a chamber having acentral depending exhaust duct and an inlet duct disposed to dischargeagainst said exhaust duct, a screen sealed to and completely surroundingsaid exhaust duct and forming said chamber into two portions isolated bysaid screen, a cathode on the. exhaust duct side of said screen, and afilm of deposited metal on the walls of said chamber on the inlet sideof said screen.

10. The method of determining the varying percentage of one gas in anotherwise substantially constant gas mixture,- comprising formingpositive ions in said gas mixture by subjecting said gas to electronsthermally ejected from a hot cathode, and measuringthe current conductedby said ions at a given potential, and repelling positive ions of saidgas cracked by contact with the cathode back toward the cathode andevacuating the same before they can conduct said current.

11. An ionization tube comprising in combination a chamber having anexhaust duct and an inlet duct, an accelerator screen in said chambercompletely separating said ducts, a cathode in said chamber on theexhaust duct side of said screen, apositive ion collecting element insaid chamber on the inlet duct side of said screen, and means to heatsaid cathode.

12. An ionization system comprising an ionization tube comprising incombination a chamber having an exhaust duct and an inlet duct, a vacuumpump disposed to draw gas from said exhaust duct, means to supplypredetermined amounts of gas to said inlet duct, an accelerator screenin said chamber completely separating said ducts, a cathode in saidchamber on the exhaust duct side of said screen, a positive ioncollecting elementin said chamber on the inlet duct side of said screen,and means to heat said cathode.

13. An ionization system comprising an ionization'tube comprising incombination a chamber having an exhaust duct and an inlet duct, a vacuumpump disposed to draw gas from said exhaust duct, means to supplypredetermined amounts of gas to said inlet duct, a screen in saidchamber completely separating said ducts, a cathode in said chamber onthe exhaust duct side of said screen, a positive ion collecting elementin said chamber on the inlet duct side of said screen, means to maintainsaid screen positive relative to said cathode, means to maintain saidelement negative relative to said screen, and means to heat saidcathode.

THOMAS D. MORGAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 674,840 Hoffman May 21, 1901930,989 Richards Aug. 10, 1909 1,212,163 Tigerstedt Jan. 9, 19171,334,143 Dushman Mar. 16, 1920 1,421,720 Roberts July 4, 1922 2,127,229McRae Aug. 16, 1938 2,149,045 Farnsworth Feb. 28, 1939 2,400,940McCollum May 28, 1946 2,454,564 Nelson Nov. 23, 1948 2,455,437 Nagel etal. Dec. 7, 1948 2,468,261 Hillier Apr. 26, 1949 FOREIGN PATENTS NumberCountry Date 586,594 Great Britain May 20, 1941 OTHER REFERENCESThermionic Vacuum Tube by Van Der Bill, pages 375, 376, 377.

General Electric Review, June 1946, pages38 to 42, article by VanValkenberg.

1. THE PROCESS OF INDICATING A CHANGE IN THE COMPOSITION OF A GAS WHICHCOMPRISES PASSING A SAMPLE OF THE GAS AT A CONSTANT LOW PRESSURE THROUGHA ZONE WHERE IT IS SUBJECTED TO A BOMBARDMENT OF SLOW ELECTRONS TO FORMA FIRST GROUP OF POSITIVE IONS AT A RATE DEPENDENT UPON THE COMPOSITIONOF THE GAS, COLLECTING SAID FIRST GROUP OF POSITIVE IONS ON A NEGATIVELYCHARGED PLATE, MEASURING THE ELECTRIC CURRENT RESULTING THEREFROM, ANDPROTECTING SAID FIRST GROUP OF POSITIVE IONS FROM CONTAMINATION WITH ASECOND GROUP OF POSITIVE IONS FORMED FROM SUCH PORTIONS OF SAID GAS ASCOME IN CONTACT WITH THE SOURCE OF SUCH ELECTRONS AND CRACK THEREON BYREPELLING SAID SECOND GROUP OF POSITIVE IONS WITH A POSITIVELY CHARGEDSCREEN.
 4. IN A GAS ANALYZER AN EVACUATED CHAMBER, AND MEANS TO REGULATETHE PRESSURE IN SAID CHAMBER COMPRISING VACUUM PUMP MEANS FOR EVACUATINGSAID CHAMBER TO A PREDETERMINED PRESSURE, AND MEANS SUPPLYING GAS TOMAINTAIN SAID PRESSURE COMPRISING A SOURCE OF SAID GAS AT GREATER THANATMOSPHERIC PRESSURE, A CONDUIT CONNECTING SAID SOURCE AND SAID CHAMBER,A CONSTANT PRESSURE REGULATING VALVE IN SAID CONDUIT RECEIVING GAS FROMSAID SOURCE AND PASSING IT UNDER REDUCED PRESSURE THROUGH SAID CONDUIT,AND A RESTRICTED ORIFICE CONTROLLING FLOW FROM SAID CONDUIT INTO SAIDEVACUATED CHAMBER, SAID CHAMBER HAVING A CENTRAL DEPENDING EXHAUST DUCTAND AN INLET DUCT DISPOSED TO DISCHARGE AGAINST SAID EXHAUST DUCT, ASCREEN SEALED TO AND COMPLETELY SURROUNDING SAID EXHAUST DUCT ANDFORMING SAID CHAMBER INTO TWO PORTIONS ISOLATED BY SAID SCREEN, ACATHODE ON THE EXHAUST DUCT SIDE OF SAID SCREEN, AND A FILM OF DEPOSITEDMETAL ON THE WALLS OF SAID CHAMBER ON THE INLET SIDE OF SAID SCREEN.